The need is known, in numerous sectors such as, for example, the pharmaceutical and food sectors, to have compressed air or technical gases too, with a high quality level, which, in general, cannot be obtained by directly using a stream of ambient air which has been previously compressed.
It is therefore necessary to perform some operations to purify the air, with the main objective of eliminating humidity, thus obtaining substantially dry compressed air, that is, almost completely without humidity.
Generally, the humidity is removed by dehumidification operations, to make available a stream of compressed air that is re-usable depending on the specific field of application.
Currently, in the field of dehumidification processes for compressed air, (which, due to conceptual and structural differences, must not be compared with systems for purifying streams of air containing noxious products such as oil, dust, percolates, etc.), different types of dryers are available, such as for example cooling dryers, adsorption dryers and hybrid types, that is, a mixture of the previous two categories, characterized by the different degrees of dehumidification obtainable.
Cooling dryers exploit the principle of humidity condensation, obtained by lowering the temperature of the compressed air in a traditional cooling cycle with a heat exchanger.
One disadvantage of this type of dryers is the presence of a lower functioning limit of the dryer, relating to the minimum dew point temperature reachable, the same as freezing temperature.
In a range of temperatures near to this, in fact, there is a risk of ice forming during the condensation of the humidity, with a possible consequent breakage of the heat exchanger due to the increase of the volume of humidity, in which the compressed air to be treated is made to circulate.
For this reason, cooling dryers are not used in applications that require dry compressed air or that has a particularly reduced humidity content, since the performance associated with them directly depends on the minimum dew point temperature obtainable.
Adsorption dryers remove the humidity by using a porous and hygroscopic material, such as for example silica gel, sifters or activated alumina, able to selectively bond with the water molecules present in the compressed air.
In this way the extraction and retention is obtained of the humidity from the stream of compressed air by physical and/or chemical processes without cooling, thus avoiding the disadvantages connected to cooling dryers.
One advantage of adsorption dryers concerns the high performance associated with them, and the possible application even in places with a cold climate, where the temperatures can be incompatible with using cooling dryers.
On the contrary, the biggest disadvantage of this type of dryer is that they need an operation to regenerate the adsorbent material because it gets progressively saturated; this can be achieved, for example, using the so-called cold regeneration technology, where part of the stream of dry air is collected at exit, generally about 15%.
Alternatively, according to the principle of hot regeneration, it is possible to regenerate the adsorbent material by using resistances and blowers, suitable to heat the adsorbent material and to determine the evaporation of the humidity bonded with the hygroscopic material.
Both solutions entail high investment and running costs (regeneration).
Another alternative is to recover the thermal stream generated during the air compression step, so as to have enough energy available to evaporate the humidity bonded with the hygroscopic material.
One disadvantage of this known solution concerns the difficulty of this application in the presence of lubricated compressors, since the lubricant itself, generally oil-based, dissipates a considerable amount of heat, reducing the overall heat stream available for regeneration.
Hybrid dryers are also known, which have only recently become widespread and which provide a first drying step through cooling, able to remove about 80-85% of the humidity, followed by a further removal of residual humidity, carried out by the adsorption technology. This method allows to considerably increase the quality level of the dry air obtainable compared with cooling dryers and, at the same time, to reduce the high running costs associated with adsorption dryers.
One disadvantage of this known solution concerns the high investment costs, since two different units are required in which to carry out the successive dehumidification steps, and also the greater running costs associated with the adsorption technology.
In the state of the art, devices are also known for purifying compressed air, which use a first air-cooling stage at a temperature higher than the freezing temperature of water, and a second cooling stage, disposed downstream and in series with the first cooling stage, in which the humidity in the compressed air is frozen and then separated from the stream. Examples of these devices are described for example in U.S. Pat. Nos. 4,976,116 and 5,428,963.
US 2012/0042691 is also known, which concerns a system to capture and recover in liquid form volatile or semi-volatile pollutant products deriving from the extraction of crude oil, to be subsequently used in non-pollutant terms. This system specifically provides to purify the stream of air of contaminants mainly consisting of hydrocarbons and it is used for underground or transportable tanks or vats for fuels. This document, in one form of embodiment, provides the combined and alternate use of two circuits, each comprising a first cooling stage with temperatures higher than the freezing temperature of water, and a second cooling stage with temperatures lower than the freezing temperature of water. The teaching of this document provides, as its main purpose, to clean the usual sources from gases arriving from storage tanks or polluted ground and which contain the gases, as well as other components, such as for example siloxanes and water.
The off gas is obtained by means of compression and condensation, possibly further treated with regenerating absorbers to remove the residual chemical vapors, after which the off gas can be re-introduced where it was extracted.
The present invention, as will be described in more detail and claimed hereafter, comes from a different field, which is the technical field of treating compressed air by cooling, in order to eliminate the water transported in the stream of air, with particular reference to the use of said compressed air in the pharmaceutical and food sectors.
In particular, one purpose of the present invention is to provide a dryer for compressed air able to work with abundantly negative dew point temperatures lower than freezing temperature, without compromising the correct functioning of the dryer.
Another purpose of the present invention is to obtain a dryer for compressed air operating with a cooling cycle, which is simple to make and able to obtain a stream of dry air with a high quality level, suitable for the most extreme applications and/or harsh climates and/or combinations thereof.
Another purpose of the present invention is to obtain a dryer connected to a distribution plant for the various user devices, which implements a method for drying compressed air able to obtain at least one partial energy recovery of the energy expended during cooling, by interchange with hot air entering and consequent heating of the dry air before it is introduced into the distribution plant for the various user devices.
Another purpose of the present invention is to obtain a dryer for compressed air that can entail lower purchase costs, running costs and energy consumption than those of both adsorption dryers and hybrid dryers of the state of the art.
The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.